Cacti are plants that can live without water for years. No plant can live entirely without water, but cacti come close. They can survive in sunny, hot, dry regions where rainfall is irregular and may not occur for months on end.
Cacti have evolved ways of collecting and storing as much water as possible when rain does come. Many cacti have long roots that grow just below the surface of the soil; this allows them to collect water from a wide area. Cacti are for the most part leafless. Plants normally lose water from pores, called stomata, on the surface of their leaves. Leafless cacti do not lose water through leaves; they have specialized pores on their stems that open only at night when the air is cool and less water can evaporate.
Most cacti have thick stems and spines instead of leaves. Some cacti are globe-shaped which allows them to hold the maximum amount of water. Other cacti have ridged or fluted stems this allows the stems to expand and store water when it rains and to contract during drought. The ribs also ensure that a full side of the plant is not constantly exposed to the sun, just portions. Most cacti have spines that reduce airflow around the plant and, in turn, evaporation. Spines help insulate the plant from heat and cold and also shade the stem from the sun. Spines protect the plant from destruction by foraging animals.
Most cacti have short growing periods and long dormancy times. This reduces the plant’s need for water during active growth. Cacti stems take on the process of manufacturing food for the plant through photosynthesis. Photosynthesis in most plants occurs in the leaves.
Cacti are native to the Americas. They can be found as far north as Canada and as far south as Chile. Most are found in sunny, hot, dry regions. There are more than 2,000 species of cacti.
The smallest cactus is Blossfeldia lilputiana, it is only 0.4 inches (1 cm) in diameter at maturity. It grows at high altitudes in the Andes mountains. The tallest free-standing cactus is the Mexican giant cardon or elephant cactus (Pachycereus pringlei); it can grow to 63 feet (19.2 m) tall and is found in northwestern Mexico. One of the best-known cacti is the saguaro (Carnegiea gigantea); it can absorb as much as 200 gallons (760 L) of water during a rainstorm. It grows in the Sonoran Desert in Arizona.